Open the door sensor, locate the reed switch, remove it and connect wires to its PCB pads. Although it should be pretty straightforward to measure the minimum distance required between the magnet and the sensor and install the magnet just a bit closer, so that any cracking of the door opens the switch. If you use a weaker magnet, the sensor will be more sensitive to the distance, so you will be able to detect smaller gaps.
Side note: if the door doesn’t close when there’s a small gap, the fridge might not be level or your door auto-closing mechanism might be worn out.